A magnetic recording medium generally and widely used these, days uses as a binder thermoplastic resins alone or in combination such as vinyl chloride and vinyl acetate type resins, vinyl chloride and vinylidene chloride resins, cellulose type resins, acetal resins, urethane resins or acrylonitrile butadiene resins. When these resina are used as a binder, there are such defects that the wear resistance of the magnetic layer is poor and that the running systems of magnetic tapes are readily stained.
It is known in Japanese Patent Application (OPI) Nos. 222433/83 and 101625/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and U.S. Pat. No. 4,492,735 that thermosetting resins such as melamine resins or urea resins are used as a binder and that a binder which is crosslinked by a chemical reaction such as an isocyanate compound or an epoxy compound can be added to the above described thermoplastic resins. However, when such a crosslinkable binder is used, there are also defects in that: (1) storage stability of the coating composition composed of ferromagnetic particles dispersed therein and resins is poor, that is, pot life is short, and uniformity of the magnetic coating composition as well as that of the magnetic tape cannot be maintained, and (2) after the magnetic coating composition is coated and dried, a heat treatment for hardening the coated film is necessary and a long period of time for manufacturing the magnetic recording medium is necessary.
In order to avoid the above defects, a method for preparing a magnetic recording medium using as a binder an oligomer and monomer of an acrylic acid ester type and hardening them by electron beam radiation after drying has been suggested and is disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 13639/72, 15104/72, 77433/75 and 25231/81. However, a magnetic recording medium having satisfactory electromagnetic properties and durability could still not be obtained in accordance with the above described method.
As a magnetic composition for a magnetic recording medium, ferromagnetic metal particles or metal alloy particles mainly composed of Fe, Ni, Co and the like have begun to be used.
That is, a magnetic recording medium is prepared by dispersing the above described ferromagnetic metal particles in an organic binder such as a copolymer of vinyl chloride and vinyl acetate, a copolymer of styrene and butadiene, epoxy resins or polyurethane resins and coating the thus prepared dispersion on a non-magnetic support.
When the above described ferromagnetic metal particles (hereinafter referred to as metal magnetic particles) are used, the magnetic recording medium using the magnetic metal particles is more suitable for higher density recording than that using oxide type magnetic particles. However, there is a defect in that the metal particles are more easily oxidized in the air than the oxide type magnetic particles. Therefore, when ferromagnetic metal particles are used in a magnetic recording medium, they are oxidized during storage, resulting in decreasing saturation magnetic flux density and in decreasing outputs. It is indispensable to granulate magnetic particles extremely finely to improve magnetic characteristics of a video tape. However, as the magnetic particles are granulated more finely, magnetic metal particles are more easily oxidized, resulting in largely decreasing the saturation magnetic flux density.
In this connection, it has been proposed to use magnetic metal particles stabilized by oxidation for a magnetic recording medium. The conventionally known methods for stabilizing magnetic metal particles by oxidation include a method of introducing oxygen-containing gas in an organic solvent and a method of gradually introducing oxygen-containing gas in an inert gas, as disclosed, for example, in Japanese Patent Publication No. 28961/81 and Japanese Patent Application (OPI) Nos. 69301/81 and 216306/86.
The ferromagnetic metal particles generally include iron particles and alloy particles of iron and another metal (e.g., Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Si, Al, P, Mo, Sn, Sb, Ag and the like). These ferromagnetic metal particles are prepared by the following methods, which are well known in this industry.
(1) A method which comprises heat decomposing an organic acid salt of a ferromagnetic metal and reducing it with a reducing gas.
(2) A method which comprises reducing acicular oxyhydroxide, acicular oxyhydroxide containing metals or acicular iron oxide obtained by heating the oxyhydroxide in the presence of a reducing gas.
(3) A method of heat decomposing metal carbonyl compounds.
(4) A method of evaporating ferromagnetic metal in the presence of an inert gas under low pressure.
(5) A method of reducing metal salts capable of forming a ferromagnetic composition in an aqueous solution of the metal salts with a reducing substance such as boron hydride, hypophosphite or hydrazine.
(6) A method which comprises electrodepositing ferromagnetic metal particles using a mercury cathode and separating the ferromagnetic metal particles from mercury.
These metal particles obtained in accordance with the above methods are immediately oxidized when exposed to air, which is undesirable. Therefore, an oxidation method of gradually oxidizing the surface of a substance to form an oxide layer around the substance for stabilization is necessary. The gradual oxidation includes a method which comprises soaking the metal particles in an organic solvent under an inert gas and evaporating the solvent in air for drying and a method which comprises introducing a mixture of oxygen having a low oxygen partial pressure and an inert gas into the same inert gas, gradually increasing the oxygen partial pressure and finally introducing air. However, sufficiently oxidation-stabilized magnetic metal particles still cannot be obtained in accordance with the above methods.
Recently, higher image quality has been required for a magnetic recording medium. To realize the high image quality, it is necessary that a surface of the magnetic layer closely contact with the video head and the audio head. In this connection, not only surface smoothness of the magnetic recording medium but also removal of foreign substances is necessary. Most foreign substances are powders of the magnetic recording layer that have peeled apart from the support while the magnetic recording medium is being prepared or is running. To prevent the magnetic layer from peeling apart, it is necessary to strengthen the adhesive force between the magnetic layer and the support. On the other hand, as the surface smoothness of the magnetic layer increases, friction at running systems of the video tape recorder increases and running tension also increases. Under such harsh conditions, running durability is more and more demanded for the magnetic recording medium. In accordance with conventional methods, a magnetic recording medium having sufficient surface smoothness, good adhesiveness between the support and the magnetic layer and good running durability cannot be obtained.
Further, although gradual oxidation is carried out, sufficient stability for oxidation cannot be obtained, and particularly under conditions of high humidity and high temperature, demagnetization takes place. It is necessary to reduce the particle size in order to increase C/N. However, as the particle size of the ferromagnetic metal particles decreases, oxidation resistance decreases.
Accordingly, it is greatly desired to develop ferromagnetic metal particles having improved C/N and excellent oxidation stability under conditions of high temperature and high humidity.
In this connection, the inventors of the present invention studied the use of thermoplastic resins and thermosetting resins, the addition of binders which are crosslinked by chemical reaction, the use of binders which are hardened by crosslinking of electron ray exposure and the use of magnetic metal particles as magnetic particles to avoid the defects that the conventional magnetic recording medium have, and as a result of thorough investigations, the inventors of the present invention have attained the present invention.